Process for forming multilayer lift-off structures

ABSTRACT

A process is disclosed for forming multilayered polyimide structure from negative photosensitive polyimide precursors. An initial polyimide layer is deposited and imagewise exposed. The unexposed portions of the initial polyimide layer are inhibited and then a second polyimide layer is deposited and likewise imagewise exposed. The films are developed, thereby forming a multilayer polyimide structure. After formation of the multilayer polyimide structure, a conductive material is applied on a substrate and then the polyimide layers are lifted off thereby forming a desired pattern of metallization.

TECHNICAL FIELD

The invention relates to a method for forming multilayer lift-offstructures and more particularly to forming multilayer lift-offstructures with negative photosensitive polyimide films and also to animproved lift-off process for forming a metallization pattern.

BACKGROUND

In the fabrication of semiconductor devices it is often desirable notonly to form patterned conductive layers but to do so such that theconductive layer has a shape which enhances electrical contact. Forexample, rounded chip pads (i.e. "solder balls") enhance electricalcontact with other metallization levels. Although there are many knownmethods for forming a patterned conductor layer on a substrate the twomost common methods of forming such a layer are subtractive etching andlift-off techniques. In subtractive etching, after a blanket conductorlayer is deposited on the substrate the layer is selectively etched inorder to remove undesired portions thereof. In lift-off, a layer(typically an insulator such as a polyimide or a photoresist) isdeposited on a substrate, and is patterned through a photomask. Theconductive layer is then deposited on the patterned insulator and theinsulator is removed from (i.e. "lifted off" of) the substrate, takingwith it the undesired portions of the conductive layer. Of these twotechniques, it has been found that lift-off is more desirable since thesolvents used to remove the insulator in lift-off cause less damage tothe underlying substrate than do the etch processes (e.g. a plasma etchor a reactive ion etch) used in subtractive etching. Also, the conductorprofile resulting from lift-off processing minimizes step coverageproblems in subsequent conductor layers.

Utilization of lift-off techniques is already a preferred method offorming patterned conductive layers and, thus, it would be ofconsiderable advantage if these same techniques could be utilized inorder to form patterned conductive layers having varied shapes, such as"solder balls". However, creation of such conductive layers would oftenrequire the use of more complex lift-off structures and multiple layersof imaged lift-off (polyimide) layers.

An exemplary lift-off process involves coating a substrate with aphotosensitive polyimide, imagewise exposing and then developing thelayer so as to expose selected portions of the substrate. Then aconductive material is applied to form a film across portions of theremaining polyimide layer as well as the exposed portions of thesubstrate. The remaining portion of the polyimide layer is then "liftedoff" taking with it the undesired portions of the metal layer leavingonly the desired pattern of conductive material on the substrate. See,for example, U.S. Pat. No. 5,006,488 issued to Previti-Kelly on Apr. 9,1991, the contents of which are incorporated herein by reference.However, the process discussed in the Previti-Kelly '488 patent utilizesonly a single polyimide layer and is mainly dedicated to solvingproblems associated with depositing a metallization layer at hightemperatures and avoiding the use of protective barrier layers duringphotolithographic processing.

The use of multiple layers of polyimides with lift-off techniques wasdescribed in an article by Winter, "Metal Deposition with PolyimideLift-off Technique", IBM Technical Disclosure Bulletin, Vol. 17, No. 5,October 1974, page 1309, in which a first layer of polyimide ispatterned through a photoresist mask. After the metal is deposited, thephotoresist mask is removed from the first polyimide layer and a secondpolyimide layer is applied for passivation.

U.S. Pat. No. 4,606,998 issued Clodgo et al. on Aug. 19, 1986 describesa method utilizing multiple polyimide layers in a lift-off structurecompatible with high temperature metal deposition. In the Clodgo '998patent, an initial polyimide layer is deposited upon the substratefollowed by deposition of a second layer of high temperature polyimideupon the surface of the initial polyimide layer. Both polyimide layersare then heated to a temperature which is below the final curingtemperature of the high temperature polyimide and above the final curingtemperature of the initial polyimide layer. This will allow the hightemperature polyimide layer to remain soluble in common developers whilethe initial polyimide layer, which is fully imidized, becomessubstantially insoluble in common solvents. Photoresist is then applied,exposed and developed and the two polyimide layers are etched forming avia which exposes the substrate. Conductive material is then depositedacross the device, namely upon the second polyimide layer and theexposed substrate. Finally, the high temperature polyimide layer (thesecond layer) is then lifted off taking with it the undesired portionsof the conductive layer while leaving the patterned conductive layer andthe initial polyimide layer which serves to passivate the conductivelayer. Although multiple layers of polyimide are used with this processit is, like the others, directed towards solving the problem ofproviding a lift-off structure compatible with a high temperature metaldeposition without the use of a barrier layer. These prior patents,although solving important problems related to the formation ofpatterned metallization layers using photosensitive polyimide films, donot deal with problems experienced when it is desirable to separatelyimage and develop overlapping layers of photosensitive polyimidematerials.

The problems experienced with creating overlapping non-congruous layersof photosensitive polyimide become readily apparent upon understandingthe properties of negative acting resists. With negative actingphotosensitive polyimide resists it is the unexposed (uncross-linked)portions of the layer which are initially removed to create the desiredpattern. For example, when a negative acting photosensitive polyimideprecursor is exposed to a predetermined pattern of radiation, such as UVlight, the exposed portions undergo cross-linking, making them insolublein a developer. Thus, upon application of the developer the unexposed(non-cross-linked) polyimides are selectively removed, creating adesired pattern for depositing a subsequent conductive film.Accordingly, imagewise exposing and developing multiple layers havingincongruous patterns is problematic since activating energy passesthrough the second layer and causes cross-linking in the underlying(previously unexposed) polyimide layer. Thus, portions of the initiallayer, which were intended to be removed by the developer, have nowbecome insoluble due to exposure (cross-linking) to the activatingenergy. This has the effect of altering or destroying the desiredpattern to be formed by the initial layer.

Thus, there exists a need for a method of forming multilayer lift-offstructures and a method of forming a patterned conductive layer havingcomplex configurations.

SUMMARY OF THE INVENTION

The present invention encompasses, in one aspect, a process for forminga multilayer polyimide structure comprising:

applying a first layer of negative photosensitive polyimide precursor ona substrate;

imagewise exposing said first layer;

inhibiting the unexposed portions of said first layer;

applying a second layer of photosensitive polyimide precursor over saidfirst layer;

imagewise exposing said second layer, wherein an uncross-linked area ofsaid second layer at least partially overlaps the inhibited portion ofsaid first layer; and

developing said first and second layers, thereby forming a multilayerpolyimide structure.

In a further aspect of the invention, inhibiting the unexposed portionof the first layer may comprise incorporating a dye, such as1-(2pyridylazo)-2-naphthol!, into the unexposed portions of said firstlayer. Another aspect of the invention encompasses inhibiting theunexposed portion of the first layer by exposing said first layer to anoxidizing agent, examples being I₂, Cl₂ or O₃, such that the double bondfunctionality responsible for cross-linking is altered.

In a further aspect of the invention, a method for forming a pattern ofconductive material on a processed semiconductor substrate, comprises:

applying a first layer of negative photosensitive precursor to saidsubstrate, wherein said precursor is a material which fully imidizes ata temperature in excess of the temperature at which said conductivematerial is to be applied;

imagewise exposing said first layer;

inhibiting the unexposed portion of said first layer;

applying a second layer of negative photosensitive polyimide precursorover said first layer;

imagewise exposing said second layer, wherein the unexposed portion ofsaid second layer at least partially overlaps said nonexposed portion ofsaid first layer;

developing said unexposed and inhibited portions of said first andsecond layers thereby forming a void in said first and second layers;

applying a conductive material within said void; and

lifting off said first and second layers.

The present invention further encompasses a multilayer structure havinga contiguous cavity, comprising:

a semiconductor substrate;

a first layer comprising a negative photosensitive material wherein saidfirst layer is positioned over said substrate and defines a firstcavity;

a second layer comprising a photosensitive material wherein said secondlayer is positioned over said first layer wherein said second layer ispositioned over said first layer and defines a second cavity that atleast partially overlays the first cavity.

In a further aspect of the invention the entirety of the second cavityoverlays the first cavity and the first and second cavities arecylindrically shaped with the first cavity having a larger radius thanthe second cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-10 are cross sectional views of a semiconductor substrate andpolyimide structure that result at various stages of the process of thepresent invention.

DETAILED DESCRIPTION

Referring to FIG. 1, the process starts by applying a layer ofphotosensitive polyimide precursor 12 to a processed semiconductorsubstrate 10. Preferably, the layer is applied at about 1000-6000 rpmfor about 15-65 seconds and then baked at a temperature of about70°-120° C. for about 10-30 minutes. Preferably the resulting film willhave a thickness of 5μ or less. Actual conditions will vary, however,depending on a number of factors, such as, for example the desiredthickness of the layer, the exact precursor material employed and soforth, as will be apparent to those skilled in the art.

While the substrate 10 is shown as being a bare silicon substrate, it isto be understood that any one of the semiconductor structures or devicescurrently manufactured in the industry (e.g. FET or bipolar transistors,storage capacitors, resistors, other active and passive electronicdevices etc.), could be arranged on the substrate and that the patternedconductor layer to be described is patterned so as to form an electricalcontact to any one or more of these structures. Substrate 10 is simplyshown as being bare merely for the purposes of more clearly illustratingthe present invention.

Numerous negative acting photosensitive polyimide precursor materialsmay be employed in accordance with the present invention. Preferably,the material is a photosensitive polyimide precursor which fullyimidizes at a temperature in excess of the temperature in which the filmof conductive material is formed, more preferably at least about250°-280° C. In a preferred embodiment, the polyimide precursor materialis a polyamic acrylate ester such as that commercially available underthe trade name "PROBIMIDE" 300 series or 7000 series manufactured by OCGCorp., and having the formula ##STR1## Such polyimide precursormaterials are described more fully in Rohte, O. et al. "High Resolution,High Photo Speed Polyimide for Thick Film Applications", Solid StateTechnology (September 1986), the entire disclosure of which isincorporated herein by reference. Other suitable polyimide precursormaterials are disclosed in Re. 30,186, issued to Rubner et al. on Jan.8, 1980; U.S. Pat. No. 4,040,831 issued to Rubner et al. on Aug. 9,1977; and U.S. Pat. No. 4,786,569 issued to Rohte et al. on Nov. 22,1988, the entire disclosures of which are also incorporated herein byreference.

Preferably high temperature polyimides which do not fully imidize attemperatures below approximately 250°-280° C. are utilized, such as apolyimide like "PROBIMIDE" discussed above. Other examples of acceptablepolyimide precursors include, but are not limited to, "PYRALIN" 2700Series, sold by the DuPont Company of Wilmington, Del. "PYRALIN" is atrademark of the DuPont Company. Another such photosensitive polyimideis sold under the trade name "PL" by the Hitachi Chemical Company Ltd.of Japan.

As may be seen in reference to FIG. 2, once applied to the substrate,the initial polyimide layer may be exposed in a predetermined pattern toradiation, such as UV light in a conventional fashion. The unexposedportions 12 of the negative polyimide precursor do not undergocross-linking and, therefore, remain soluble in common developers.However, the exposed portions 14 of the polyimide precursor undergocross-linking and become insoluble in common developers. For example,when using Probimide 349, exposure of the film to UV radiation resultsin the following reaction: ##STR2##

The unexposed portions 12 of the negative polyimide precursor may thenbe treated so as to prevent or inhibit the cross-linking reactionassociated with exposure to radiation. "Inhibiting" the unexposedpolyimide precursor refers to a process for modifying the unexposedpolyimide precursor or the functionality required for cross-linking,thereby making the polyimide incapable of being cross-linked by exposureto radiation. As shown below (and in reference to FIG. 3), inhibition ofthe unexposed polyimide layer 12 may be done by altering the double bondfunctionality responsible for the cross-linking mechanism in order toform an inhibited portion 16 which remains inactive upon exposure toradiation. Again, with reference to Probimide 349, exposure of theuncross-linked polyimide to I₂ produces the following reaction: ##STR3##Similarly, exposure of the uncross-linked polyimide to O₃ produces thefollowing reaction: ##STR4## Any reagent that removes (by oxidation,reduction, addition, etc.) the cross-linkable olefinic functionalitywithout damaging the polyimide or polyimide precursor would work. It hasbeen found that the double bond functionality is removed by exposure toozone gas, iodine vapor or chlorine gas. The necessary amount ofexposure to the oxidizing agent will vary with the particular oxidizingagent chosen, the concentration of the same, as well as other factorsknown to those skilled in the art. For example, the substrate may beexposed to iodine vapor simply by placing iodine crystals in a reactionchamber at ambient temperature and pressure for 15 minutes to 11/2hours. However, it has been discovered that exposure of theuncross-linked portions of the polyimide to the iodine vapor at thisconcentration for over 2 hours typically results in a slightly inferiorfilm.

Another method of inhibiting the unexposed/uncross-linked portions ofthe initial polyimide layer is to soak the imaged film in a solution ofdye material, such as PAN, 1-(2 pyridylazo)-2-naphthol!, in xylene. Thedye will penetrate the film and become incorporated into theunexposed/uncross-linked polyimide layer 16. It is preferable to use adye such as PAN, which absorbs significant amounts of energy at theirradiating wavelength in order to effectively inhibit and preventcross-linking caused by exposure to UV radiation. Generally, thesubstrate may be soaked in the dye for approximately 15 minutes to 2hours in order to insure sufficient permeation of the dye into theunexposed polyimide precursor. The length of time required forsufficient permeation of the dye will vary with the nature of the dyeselected, the polyimide precursor, the thickness of the film and otherfactors well known to one skilled in the art.

After inhibiting the unexposed portions of the polyimide layer a secondlayer of negative photosensitive polyimide precursor 18 is similarlyapplied over the initial polyimide precursor 12 (as seen in FIG. 4). Forsome purposes the second polyimide layer may be applied at slower spinspeeds of 100 to 1000 rpms in order to form a thicker film. In thepreferred embodiment the second layer preferably has a thickness ofabout 50 to 100μ. After deposition of the second negative photosensitivepolyimide film it is imagewise exposed, as above, in accordance with thedesired pattern. However, as can be seen in reference to FIG. 5, theselected exposure pattern of the second layer overlaps portions of theinhibited portion 16 of the first layer which do not undergo thecross-linking reactions as discussed above. Thus, the inhibited portions16 of the first polyimide layer, as with the unexposed portion 18 of thesecond polyimide layer, remain soluble in common developers.

Developer may then be applied by means well known in the art, examplesbeing submersing the substrate in the developer, spray application ofthe developer, and so forth. A preferred developer consists of a 50/50solution of xylene/butyrolactone and is generally applied for about 4-7minutes. In general, the development time will depend upon the thicknessof the layers, as well as other factors known to those skilled in theart. Application of the developer removes the unexposed portions 18 andthe inhibited portions 16 leaving the exposed/cross-linked areas 14, 19which create a multilayer polyimide structure (an example of such astructure is shown in FIG. 6).

As indicated hereinabove, the formation of these multilayer structuresmay be extremely effective in forming a pattern of metallization.Although the present invention is discussed herein with reference to apreferred embodiment, so as to more clearly illustrate the invention, itwould be readily apparent to those skilled in the art to adapt thepresent method so as to form lift-off structures having three or morelayers. Moreover, this process could be readily adapted so as to utilizea positive photosensitive polyimide as the second layer.

A multi-layer polyimide structure found to be particularly useful isshown in FIG. 6, where the second polyimide layer overhangs a cavitycreated in the developed initial polyimide layer, thereby forming aninverted "T" shape, having an undercut region 20 and a column typeregion 21. After development a baking step is generally performed inorder to drive off the developer, preferably at a temperature at least10° C. in excess of that which conductive material is to be deposited.It is important to note, however, that temperatures should not beutilized which fully imidize the polyimide precursors since this mayrender the polyimide structures insoluble in common solvents. The lengthand temperature of baking will ultimately depend on several factors,including the type of polyimide used, the type of heating element usedto heat the substrate and other factors known to one skilled in the art.Preferably, when using Probimide 349 the substrate is heated totemperatures between 150°-175° C. and allowed to remain at thistemperature for 3 to 10 minutes.

A film of conductive material may then be formed on the remainingpolyimide structures and the uncovered portions of the substrate 10.Preferably, the film of conductive material is a metal film which isdeposited by evaporation in a temperature in the range to 175° C., morepreferably in a range up to about 165° C. In a preferred embodiment, aninitial amount of conductive material, for example,chromium/copper/gold, is initially deposited by evaporation,electroplating or other means known in the art such that it forms a thinlayer 22 across the exposed substrate 10 within the undercut 20 suchthat it does not rise to a height which allows it to contact the secondlayer 19 or fill the column 21, for example a deposition ofapproximately 2μ of conductive material may be initially depositedwithin the undercut region. The evaporator is preferably set up andangled so that the conductive material is deposited evenly across theexposed substrate in the undercut, thereby forming a "button" shapedlayer. In the preferred embodiment a second layer of conductivematerial, for example lead/tin, is deposited by evaporation. However, itis preferred that the evaporator for the second deposition be angled andset up such that the conductive material is deposited so as to form asubstantially column shaped conductive layer 24. Preferably, this columnshaped conductive layer 24 is about 50 to 100μ thick. The evaporationtechniques discussed above are known in the art and referencesdescribing the same include R. Geffken, J. Ryan and G. Slusser, "ContactMetallurgy Development for VLSI Logic" IBM Journal of Research &Development, 31, 606-616 (1987), and L. Maissel and R. Glang, Handbookof Thin Film Technology, Ch. 1 (1970).

As shown in reference to FIGS. 8 & 9, after the desired metaldepositions, remaining portions of the polyimide layers 14, 19 and theportions of the conductive film 23, 25 which overlie the remainingpolyimide structure are lifted off the substrate thereby leaving themetal structures formed within the undercut and column regions. Removalof the polyimide structures may be accomplished by any one of numerousmethods known in the art. An example being immersion of the substrate ina solvent, such as N-methyl pyrrolidone (NMP) at about 70°-90° C. forabout 1-3 hours. Immersion in NMP is preferably accompanied byagitation, such as N₂ bubbling. However, in the preferred embodiment,the conductive material upon the polyimide structures is first removedby application of a tape, an example being tape No. 850 which is soldand manufactured by the 3M Corporation of Minneapolis, Minn., followedby removal of the polyimide structures and any remaining conductivematerial thereon by application of hot NMP.

After removal of the polyimide structure, the substrate is heated to atemperature sufficient to melt the column shaped conductive layer 24,such as 365° C. to 390° C. Due to wetting between the thin layer ofCr/Cu/Ag and the Pb/Sn the Pb/Sn will remain upon the Cr/Cu/Ag film andform a substantially rounded shape as shown in FIG. 10.

EXAMPLE 1

A semiconductor structure was fabricated by spin application (dynamicapply 15 s/500 rpm 15 s/3500 rpm-30 s/6000 rpm) of a negativephotosensitive polyimide precursor ("PROBIMIDE" 349), followed by bakingfor 30 minutes by ramping the temperature to 110° C.(80°-80°-80°-110°-110°-110°-110.degree. C. /30 total minutes), therebyforming a polyimide layer approximately 2μ thick. The "PROBIMIDE" 349material was then exposed to a dose of 360 mJ/cm² at 438 nm using amercury lamp (G-line stepper 200 ms at -2.8 focus) leaving unexposed acircular region having a radius of approximately 100μ. The substrate wasthen placed in a chamber with iodine crystals at ambient temperature andatmospheric pressure for 30 minutes and then removed. A secondapplication of negative photosensitive polyimide precursor ("PROBIMIDE"349) was spin applied (dynamic apply 15 s/500rpm-5 s/400 rpm-45 s/200rpm) and baked to 110° C. for 30 minutes(80°-80°-80°-110°-110°-110°-110.degree. C./30 total minutes). Thesubstrate and the multiple layers of polyimide were again exposed to UVradiation as above, leaving unexposed a circular pattern of radius ofapproximately 75μ substantially centered over the previous unexposedregion. The polyimide films were then developed by spray application ofa 50/50 solution of gamma butyrolactone/xylene for 120 seconds, followedby an initial rinse with gamma butyrolactone/xylene for 30 seconds and asecond rinse for 10 seconds with xylene. The substrate was then baked ona hot plate for 10 min. at 165° C. An initial layer of metal, namelyCr/Cu/Ag, was deposited by evaporation in a vacuum chamber so as to forma thin film of about 2.5μ upon the exposed substrate within the undercutregion within the polyimide structure. The substrate was removed andplaced in a second evacuator where a second layer of metal, namelyPb/Sn, was deposited by evaporation forming a substantially columnshaped structure about 50μ thick within the undercut and column regionsof the polyimide structure. The metal layers upon the polyimidestructure were substantially removed by application of 3M tape No. 850followed by lifting off of the polyimide layer and metal remainingthereon by immersing the substrate in hot NMP at about 80° C. for 1hour. The substrate was then placed in a H₂ reflow furnace at 375° C.for 10-20 minutes wherein the Pb/Sn layer melts and conforms to the thinlayer-of Cr/Cu/Ag.

While the present invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodto those skilled in the are that various changes in form and details maybe made therein without departing from the spirit and the scope of theinvention.

What is claimed is:
 1. A process for forming a multilayer polyimidestructure comprising:applying a first layer of negative photosensitivepolyimide precursor on a substrate; imagewise exposing said first layer;inhibiting the photosensitivity of the unexposed portions of said firstlayer; applying a second layer of photosensitive polyimide precursorover said first layer; imagewise exposing said second layer, wherein anuncross-linked area of said second layer at least partially overlaps theinhibited portion of said first layer; and developing said first andsecond layers, thereby forming a multilayer polyimide structure.
 2. Theprocess of claim 1 wherein inhibiting said unexposed portion of saidfirst layer comprises incorporating a dye into said unexposed portionsof said first layer.
 3. The process of claim 2 wherein said dyecomprises 1-(2 pyridylazo)-2-naphthol.
 4. The process of claim 1 whereininhibiting the unexposed portions of said first layer comprises exposingsaid first layer to a oxidizing agent, wherein the double bondfunctionality responsible for cross-linking is altered.
 5. The method ofclaim 4 wherein said oxidizing agent comprises a halogen.
 6. The processof claim 5 wherein said halogen is selected from the group consisting ofchlorine and iodine.
 7. The process of claim 6 wherein said halogencomprises iodine vapor.
 8. The process of claim 5 wherein said oxidizingagent comprises ozone.
 9. The method of claim 1 wherein said secondlayer comprises a negative photosensitive polyimide.
 10. The method ofclaim 9 wherein said first layer and second layer are imagewise exposedwherein the unexposed portions of said first layer are larger than theunexposed portions of said second layer.
 11. The process of claim 9wherein said first and second layers are imagewise exposed wherein theinhibited portion of said first layer and the uncross-linked portion ofsaid second layer are circular in shape and the cross-linked portion ofsaid second layer substantially overlaps the inhibited portion of saidfirst layer.
 12. The method of claim 1 wherein developing said first andsecond layers comprises applying a solution of xylene/butyrolactone tosaid substrate.
 13. The method of claim 1 wherein said first layercomprises a polyamic acrylate ester.
 14. A method for forming a patternof conductive material on a processed semiconductor substrate,comprising:applying a first layer of negative photosensitive precursorover said substrate, wherein said precursor is a material which fullyimidizes at a temperature in excess of the temperature at which saidconductive material is to be applied; imagewise exposing said firstlayer; inhibiting the photosensitivity of the unexposed portion of saidfirst layer; applying a second layer of photosensitive polyimideprecursor over said first layer; imagewise exposing said second layer,wherein the uncross-linked portion of said second layer at leastpartially overlaps said inhibited portion of said first layer;developing said uncross-linked portions of said second layer and saidinhibited portions of said first layer thereby forming a void in saidfirst and second layers; depositing a conductive material within saidvoid; and lifting off said first and second layers.
 15. The method ofclaim 14 wherein said first and second layers are imagewise exposed soas to form an undercut region in said void.
 16. The method of claim 15wherein said conductive material deposited within said void is depositedso as to form a thin layer whose thickness is less than 10% of the depthof the void.
 17. The method of claim 16 wherein said conductive materialis selected from the group consisting of chromium, copper, gold andcombinations thereof.
 18. The method of claim 17 wherein said conductivematerial comprises chromium, copper and gold.
 19. The method of claim 16whereby said second layer is imagewise exposed so as to form a voidhaving a column shaped region connected to said undercut region.
 20. Themethod of claim 14 further comprising depositing a second conductivematerial within said void prior to lifting off said first and secondlayers.
 21. The method of claim 20 further comprising depositing asecond conductive material within said void prior to lifting off saidfirst and second layers whereby said deposited second conductivematerial is substantially column shaped.
 22. The method of claim 21wherein said second conductive layer comprises Pb and Sn.
 23. The methodof claim 14 wherein lifting off said first and second layers comprisesapplication of N-methyl pyrrolidone.
 24. The method of claim 23 wherein,prior to lifting off said first and second layers, placing acrylicadhesive of a tape in contact with the conductive material over saidsecond layer and then removing said tape.
 25. The method of claim 14wherein prior to depositing said conductive material the substrate isbaked at a temperature at least 10° C. in excess of the temperature atwhich the conductive material is deposited but below a temperature whichfully imidizes the polyimide precursor.